Combined Perturbations on Semiconductor Lasers for Photonic Millimeter-Wave Generation

Abstract

Photonic generation of millimeter-wave (mm-wave) signals has long been attractive for applications due to the broad optical and electrical frequency ranges, availability of low-loss fiber-based transmission, elimination of high-frequency electronics, and potential of integration. These signals ranging from narrowband tones to broadband chaos are promising for a number of applications in wireless communications, radar systems, machine learning, and signal processing. Amongst the approaches of photonic mm-wave generation, the utilization of the inherent nonlinear dynamics of semiconductor lasers is advantageously versatile because of the possibility of employing a variety of perturbations. In this thesis, combinations of perturbations on semiconductor lasers are investigated for generating different signals in reaching the mm-wave band. The combined perturbations include cascaded injection with external locking, slow intensity modulation with fast phase modulation, and detuned filtered feedback with dispersion. They are adopted for invoking various forms of nonlinear dynamics in the single-mode lasers that extend to the mm-wave band. Firstly, a semiconductor laser perturbed by the combination of cascaded injection and external locking is explored for mm-wave generation with phase noise suppression. The experiment using an optical injection from a master laser induces the period-one (P1) oscillation at a fundamental frequency ƒ₀ = 36 GHz in a primary slave laser, which in turn injects a secondary slave laser to enhance the P1 oscillation harmonics. Such a cascaded injection alone results in iphotonic mm-wave generation at the doubled frequency of 2ƒ₀ = 72 GHz at the V-band. The experiment then incorporates to the injection of the primary slave laser a phase modulation from a microwave source at merely ƒ₀/4 = 9 GHz for external locking. The resultant output mm-wave at 2ƒ₀ = 72 GHz is frequency-stabilized, where the single-sideband phase noise is suppressed to −87 dBc/Hz at 10-kHz offset. Strengthening of the mm-wave power is also demonstrated via a coherent addition from the master laser. Secondly, a combination of a slow intensity modulation and a fast phase modulation is applied to the injection of one laser in P1 oscillation for achieving locked frequency-modulated continuous-wave (FMCW) mm-wave generation. The slow intensity modulation causes adiabatic sweeping of the output frequency, while the fast phase modulation provides seeding for locking the whole comb. An inherent competition between such slow and fast perturbations is unveiled, where an optimal locking strength is identified. An FMCW signal with a sweep range of 6 GHz and an enhanced comb contrast of 42 dB is experimentally obtained, where the central frequency is boosted to 80 GHz by a coherent addition with another externally locked laser. Numerical simulation results based on a rate-equation model are found to be in qualitative agreement with the experiments. Additionally, filtered optical feedback with dispersion is investigated through using a fiber Bragg grating (FBG) that is detuned to match a residual side mode of the single-mode distributed feedback laser. With the laser mode spacing being slightly greater than the grating bandwidth, the main mode is suppressed while the residual side mode is selectively excited. The residual side mode is found to be in broadband chaotic dynamics, where the electrical bandwidth reaches 26 GHz in corresponding to a broadening of over 50% as compared to the main mode of the laser. Wavelength tuning beyond 10 nm is allowed by selecting different residual side modes through changing the gratings. The resulting broadband dynamics is nonetheless entirely described by the one selected mode without involving complex interactions between multiple modes. Overall, the combined perturbations are promising for extending the applications of single-mode laser dynamics to the mm-wave band.

Date of Award	17 Apr 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Sze Chun CHAN (Supervisor)